Probably won't let you grab it unless you register and pay for the site separately when you get there.
As I remember, they have you go in and pick a site, do all the paperwork and pay for it before you go drive around looking for empty sites. If you decide on a different available site, you have to go through the entire process again, e.g. paperwork and payment (different sites have different rates). Seems kind of dumb to me, but that is how they do it.
Most Alabama State Parks require/want some equipment set up on the site immediately, so someone else driving through doesn't think the site is vacant and try to register for it.
A reservation really doesn't guarantee you anything other than that they will have a space available (out of 500) when you come in. Makes it sort of tough if you are trying to get your friends on the site next to yours.
Great park. We will probably be there mid to late June.